The present invention relates to the detection and imaging of infrared radiation and, more particularly, to a device, for detecting and imaging infrared radiation, that facilitates the alternation among several uncooled filters in conjunction with a cooled detector of the infrared radiation.
It is well known in the art of thermal imaging infrared sensors based on cooled detector arrays that in order not to flood the detector with unwanted spurious self-emission from the environment two design elements must be present in the system: i) the collecting optics (lenses or mirrors, filters and windows) must be made of non-absorbing infrared transmitting materials in the wavelength range of sensitivity of the detector (so as not to emit appreciable amounts of infrared radiation introducing noise and masking the radiation to be detected), and ii) the entrance pupil of the collecting optics must be imaged on a cold shield aperture present in the cryogenically cooled space inside the detector Dewar, again in order to avoid radiation from the environment to introduce noise and spurious signals. These design rules are necessary because the optical elements, the optics housing of the system as well as the environment emit a large amount of radiation in the infrared range which in general masks the radiation to be detected and originating in the scene to be analyzed. However in many cases and situations, in addition to the imaging optics and windows, there is the need to use a number of spectral filters placed alternately on the optical train in the sensor system, in order to detect and identify and recognize different sources placed in the field of view of the system. These filters may be narrow-band, wide-band, cut-on, cut-off or otherwise spectrally limiting the incoming radiation from the scene, so that these objects can be detected and identified based on their spectral characteristics.
As a result, since a spectral filter may be a source of self-emitted infrared radiation in its own right and may reflect environment radiation into the detector if simply placed in the collection optics train, the common knowledge and practiced art is to place the filter inside the Dewar so as to be cooled to cryogenic temperatures: this insures minimizing the self-emission of the filter and preventing the filter from introducing spurious radiation into the detector. The main disadvantage of this filter cooling method is that once the filter is built into the Dewar it cannot be exchanged for a different one, preventing the possibility of acquiring different successive images of the scene to be analyzed through different narrow wavelengths or through different spectral ranges so that the spectral capability of the system is very limited.
The purpose of the present invention is to provide infrared sensor systems with the advantages of the prior art systems that use cooled detectors and a cooled filter but with the significant advantages of: i) enhanced spectral capability by being able to use a succession of spectral filters or a continuously variable filter (CVF) or a different type of filter plurality, ii) avoidance of cooling the filters, which results in a simpler and less expensive system.
As it is taught in U.S. Pat. No. 5,434,413 to Kennedy, optical filtering in prior art systems based on cooled photon array detectors can be done for example in the following two ways: i) by placing the filter in direct contact with the cooled detector (see Kennedy FIGS. 1b and 1d) so that the filter itself is cooled and its self-emission is very small, and as a consequence its contribution to signal and background noise is minimized; ii) by placing a bandpass filter coating on the vacuum window (see Kennedy FIG. 2), this window being constructed of a non-absorbing material (see Kennedy, end of column 3): the fact that the window is made of non-absorbing material insures here too that the self-emission of the window and as a consequence its contribution to background noise is minimized even if the window is not cooled. In both configurations of the prior art of Kennedy the filter is constructed as a physical part of the vacuum vessel or Dewar, either inside the vessel or being coated or attached on its window. Both these configurations have the following disadvantages (besides the ones mentioned in that patent for the former configuration).
A) These configurations do not allow the use of more than one filter in the system in succession for image detection in more than one spectral range or more than one wavelength (this being the spectral range or the coating of Kennedy FIGS. 1d and 2): in this case, for example, when more than one narrow band signal is needed from each pixel of the image to be measured the filters must be used outside the Dewar on the optical train of the telescope and therefore in general, absent the special innovative improvements of the present invention as described below, the filters will have to be enclosed in an additional vacuum vessel and be cooled in order to avoid their own self-emission.
B) Even with this additional cooling of the filters, reflection of background radiation towards the detector in the unwanted noise contributing spectral range cannot in general be avoided.
C) The filter configuration inside the Dewar of Kennedy FIGS. 1b and 1d usually requires special work from the detector manufacturers because the filter, being application dependent, is not standard and as a consequence a very high price is paid for the Dewar and detector construction.
U.S. Pat. No. 3,770,958 to Krakow teaches the use of a single filter and the use of a series of interchangeable non-emitting uncooled filters placed in front of the Dewar window in a system using a single or multiple stacked detectors (not an imaging array). It is not obvious that such arrangement can be easily extended to the case of imaging array detectors. In fact, in the case of the single filter often there is not enough physical space in front of the window for the filter to be close enough to the Dewar window so that the edge of the array will not receive spurious radiation from the surroundings. The case of a rotating multiple filter wheel in that patent requires a lens to be placed inside the Dewar to be cooled: this is a very expensive and cumbersome proposition.
U.S. Pat. No. 5,408,100 to Gallivan teaches an uncooled non-emitting filter that is a multilayer coating, on the last spherical concave surface of a lens in the optical system, whose radius of curvature is equal to the distance of the surface to the detector. This arrangement produces in general an unwanted ghost image of the array pixels superimposed on the desired image provided by the system due to the fact that the pixels and the surface separating them usually have different reflectivity.
It would be highly advantageous to be able to use more than one filter in the same system for comparison of signals from different spectral ranges and to use a standard infrared cooled-array-camera-based sensor system without an expensive and cumbersome cooling system needed for any of these filters or lenses and with minimal loss of signal to noise ratio or dynamic range in spite of the filters not being cooled.